The present invention relates to a circuit arrangement for apparatus capable of receiving a color television signal, coded according to a system in which the chromatic information modulates in amplitude by carrier suppression a subcarrier along two axes in quadrature, one of the axes being switched 180.degree. each line, a single reference signal of constant phase serving as a reference signal for the demodulation of said subcarrier and as an identification signal for said switching, said single signal being inserted only every second line; said circuit arrangement being arranged to process the chrominance signal in order to provide the identification signal for said switching.
Such a color television system (ISA system) constitutes an improvement of the PAL system (Phase Alternative Line) in which, as well known, the chromatic information modulates (with suppressed carrier), in amplitude and phase, a color subcarrier. In said PAL system, the modulation direction (phase) at alternate lines is overturned with respect to a fixed axis; and there is further inserted, during the blanking period of each line, a reference signal (burst) constituted by ten oscillations at the subcarrier frequency, whose phase is also alternating, of 45.degree. with respect to said fixed axis.
In practice, the ISA system has been proposed in two forms, which consist:
THE FIRST, IN THE SUPPRESSION OF SAID REFERENCE SIGNAL DURING THE LINES IN WHICH THE PHASE OF THE CHROMATIC INFORMATION IS OVERTURNED; THIS VARIANT IS INDICATED, IN THE CONTINUATION OF PRESENT SPECIFICATION, BY THE NAME OF ISA-P system;
THE SECOND, IN THE INSERTING OF SAID REFERENCE SIGNAL WITH A PHASE CORRESPONDING TO THE FIXED AXIS (THAT IS, WITH THE SAME PHASE USED IN THE NTSC system) but only in the lines wherein the chromatic information is not inverted; in the continuation of present specification, this variant is indicated by the name of ISA-N system. (ISA for Identity Suppressing Alternately).
When a signal, according to any one of the mentioned systems is received, it is usual to provide two synchronous demodulators for obtaining the two "color difference" signals commonly indicated as B - Y and R - Y signals. A persistent oscillation at subcarrier frequency having the phase of the fixed axis, is forwarded to the first of said demodulators; a quadrature oscillation whose phase is inverted at line frequency, by means of a change-over switch, under the command of a switching voltage generator, is forwarded to the second.
In order to synchronize the switching voltage generator, it is usual in the
system to provide a phase comparator which compares the oscillation produced with the arriving reference signals, and thereby produces a signal at 7.8 Hz (half line frequency) which is used as a signal to identify the switching of the generator.